A Multi-Proxy, Paleomonsoon Record Based on Speleothems From Central China

The oxygen isotopic composition of cave calcite deposits (speleothems) formed through equilibrium processes is a function of cave temperature and drip-water composition, which is closely related to precipitation. Oxygen isotopic variations in speleothems from mid-to-high latitudes have been widely used as isotopic paleothermometers, because of the strong relationship between δ¹⁸O of precipitation and surface temperature in these regions. Interpretation of speleothem records from lower latitudes or from regions with complex atmospheric circulation patterns, such as the Asian monsoon region, is much less straightforward. In regions affected by the Asian monsoon, past variations in δ¹⁸O and δD of precipitation have been attributed to variations in monsoon intensity, storm tracks, and/or variations in temperature. We have investigated the strength of the transfer functions, dδ¹⁸O_p/dT and dδ¹⁸O_p//dP, in China using multiple regression analysis of data from the Global Network for Isotopes in Precipitation (GNIP). We determined that in areas which experience a strong summer monsoon, the amount effect is dominant, i.e. δ¹⁸O is lowest in the summer. In areas that are less influenced by the summer monsoon, temperature is the dominant control on δ¹⁸O and, hence δ¹⁸O is lowest in the winter. The slopes of these relationships, however, vary spatially and temporally as monsoon intensity changes. Therefore, use of oxygen isotopic variations in speleothems from China as a quantitative paleotemperature or paleoprecipitation proxy may not be valid. We present oxygen isotopic records from 3 stalagmites collected from Wanxiang Cave, Gansu Province, China (33.31° N, 105.00° E.) The speleothems are well dated by TIMS U-series analyses. Favorable (²³⁰Th/²³²Th) of 350-3000 result in only very minor detritus corrections. Ages preserve stratigraphic order and range from approximately 20-313 ka. Several depositional hiatuses throughout this period correspond to global stadial periods when speleothem growth stopped in Wanxiang Cave, most likely due to decreased precipitation. Our isotopic calibration shows that modern speleothem calcite forms in equilibrium and that there is no significant kinetic fractionation in Wanxiang Cave. At this site, oxygen isotopes in precipitation, and hence in the speleothems, are inversely related to temperature. Significant shifts are seen throughout the records, reflecting high and low amplitude changes in monsoon intensity, some of which have been documented in other paleorecords. Due to the complicated systematics of stable isotopes in precipitation here, however, we use uranium concentration and isotopic data as additional evidence. Initial (²³⁴U/²³⁸U) and U concentrations are positively correlated with δ¹⁸O in the speleothems. This lends support to the use of δ¹⁸O in speleothems as a paleoprecipitation proxy, since during drier periods, when δ¹⁸O_p is high, evaporative enrichment and increased residence time of vadose groundwaters is expected to lead to increased trace element concentrations and a higher excess ²³⁴U as a result of recoil and incongruent dissolution. Through this multiproxy approach to paleoclimate reconstruction, there is a strong potential for improvement in the understanding and, hence, applicability of stable isotopic variations in cave calcite deposits.